Nebulizers, or atomizers as they are sometimes called, are devices that generate a fine spray or aerosol from a liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or a suspended particulate drug for administration to a patient by inhalation.
Piezo-mesh based nebulizers are commonly used to generate aerosols in such drug delivery apparatus, whereby for instance a piezoelectric element vibrates a nozzle plate (otherwise referred to as a mesh) to produce the fine aerosol spray. In some nebulizers the piezoelectric element is bonded to a nozzle plate element, whereas in other nebulizers the nozzle plate element is separate from (i.e. not in contact with) the piezoelectric element (sometimes referred to as piezo-cavity-mesh based nebulizers). An advantage of having the nozzle plate element separate from the piezoelectric element is that the nozzle plate element can be removed from the nebulizer and cleaned or entirely replaced after a certain amount of use.
In order for a particular medicine to be therapeutically effective when inhaled, the aerosol droplet size of the medicine must be within a narrow therapeutic range. As the generation of droplets is accomplished by pressing the liquid through the holes (nozzles) in the nozzle plate, and the size of the droplets is determined, in part, by the size of the nozzles in the nozzle plate, ideally each nozzle in the nozzle plate should be of the required size. Typically, it is desirable for the nozzles to have a diameter of 2.5 μm with a tolerance of +/−0.25 μm. There can be of the order of 5000 nozzles in a typical nozzle plate.
Due to limitations in the process of manufacturing nozzle plates, it is difficult to consistently produce nozzle plates with nozzles at the desired size (or within the required tolerance) and with the nozzles having a consistent size across each plate, and thus the production yield of the manufacturing process is quite low, sometimes around 10%.
Furthermore, it has been found that even a small variation in nozzle size (for example within the normally accepted tolerance range for nozzle size of +/−0.25 μm) leads to a large variation in the output rate (flow rate) of the nebulizer. Since the dosage regime for a particular medicine may need to be precisely controlled, this variation in flow rate is unacceptable. In particular, to achieve a variation in flow rate of just 25% from a desired flow rate, it has been found that the maximum permissible variation in nozzle size from the desired size is +/−0.05 μm. However, applying this smaller tolerance range to the conventional manufacturing process would further reduce the production yield. As the nozzle plate is a consumable element of the nebulizer (i.e. it can often be replaced), this is highly undesirable.
Therefore, there is a need for an improved way of operating a nebulizer to achieve a desired flow rate and/or droplet size, despite the variation in nozzle size across different nozzle plates that could be used in the nebulizer.